1. The Field of the Invention
The present invention is directed to color filters and methods for preparing color filters. In particular, the present invention is directed to color filters for use in visual displays and methods for preparing the same.
2. The Relevant Technology
Color filters are used to produce full color images in visual displays. The three primary colors, red, green and blue used to produce full color images in projection displays, flat panel displays and other visual display devices are most commonly provided by color filters.
Generally, color filters consist of a transparent substrate having a repeating pattern of pixels on its surface. Pixels are defined as the smallest controllable area on visual displays, having the same color that are capable of being located and turned "on" and "off" by a computer display. Each pixel on a color filter is associated with a primary color, and is arranged with other color pixels in repeating arrays of red, green and blue triads. Depending upon which pixels light is passed through, color filters containing pixels of the three primary colors are capable of producing color images of a wide variety of colors.
Although the use of color filters to produce full color images has long been known, color filters continue to occupy the highest proportion of material costs in visual displays, such as flat panel displays. This expense can be attributed to a number of factors, such as color filter material costs and the number of manufacturing steps; however, the high cost of color filters is primarily due to low yields observed in color filter manufacturing processes.
Yields in color filter manufacturing process are determined by the number of color filters produced that meet industry standards out of the total number of filters produced. Panels not meeting industry standards of approximately 3 to 4 defects per ten inch diagonal must be discarded thereby reducing color filter manufacturing yields and resulting in high production cost and low consumer availability. A common defect and major contributor to the low yields presently experienced in the industry is the omission of pixels, a phenomenon commonly referred to as "drop out." When "drop out" occurs, uncolored, unfiltered light passes through the filter to the eye of an observer as opposed to the color intended to be produced.
Originally, color filters were prepared by a dyeing, or gelatin process in which a layer of gelatin or other dyeable material formed on the interior of a transparent substrate was colored using photolithography techniques. More recently, polyimide systems comprising thermally stable dyes combined with polyimides, have been incorporated into photolithography processes to improve filter quality. Although color filters prepared using photolithography exhibit good resolution and color quality, photolithography is labor intensive and results in poor yields. For example, photolithography requires that each color incorporated onto the filter have a mask, a photoresist, baking and etching steps, and resist removal. To produce a color filter having the three primary colors, this process must be repeated three times.
Because of the complexity of the photolithography process and the tedious steps that must be repeated for each primary color, photolithography processes have consistently given unsatisfactory yields, typically on the order of 50%. Even after repairing defective color filters by repeating the photolithography process for omitted pixels, the yield only increases to approximately 70%. Taking into consideration the cost of repair and the high percentage of filters that remain unusable, there has for some time existed a need for a color filter manufacturing process having greater yields and consequently producing color filters at lower costs.
More recently, in an attempt to overcome the above-mentioned deficiencies, a dye diffusion process for making color filters has been proposed. In this process, a sublimable dye is transferred from a donor sheet containing the dye to a polymeric receiver sheet which becomes the color filter. An exemplary dye diffusion process is disclosed in DeBoer et al. U.S. Pat. No. 4,965,242. In DeBoer et al., a dye-donor element is placed over a dye receiving element, wherein the dye receiving element comprises a temporary support having thereon a polymeric alignment layer, transparent conducting layer and a polymeric dye receiving layer. Heat is applied to the donor element by radiation energy means, such as a thermal printing head or heat absorption by infrared dyes, causing the dye image to be transferred from the donor sheet to the receiver sheet. Once the transfer has occurred, the dye donor sheet is replaced with a glass support to form the color filter. Other patents disclosing similar dye diffusion processes for preparing color filters include U.S. Pat. Nos. 4,962,081, 5,073,534, and 5,242,889.
Here again, although the dye diffusion process simplifies color filter manufacturing, adequate yields are still not attained. An additional drawback that adds to the low yields and increased cost of the color filter is that the dye diffusion process is presently limited to sublimable subtractive dyes, namely magenta, yellow and cyan.
Producing primary colors using subtractive dyes requires layers of magenta, yellow and cyan to be formed in various combinations to make red, blue and green additive colors. One exception is found in U.S. Pat. No. 5,242,889, issued to Shuttleworth, which discloses a blue sublimable dye. However, because the dye diffusion process must be repeated to form pixels associated with red and green, there exists a greater opportunity for a defect to occur, resulting in an increased number of unusable color filters and, consequently, the continuation of depressed yields.
As expected, the "drop out" rate in color filter manufacturing increases during mass production. In addition, as illustrated in FIG. 1, as the panel size increases, the number of "drop outs" rises dramatically. For example, a defect level of one defect per square inch sharply increases to approximately 50 defects per ten inch diagonal color filter as shown by line 30. A marked decrease can be attained by limiting the defects to 0.1 defects per square inch (line 32) and as shown by lines 34 and 36, the number of defects drops sharply when the number of defects is reduced to 1/16 of a defect per square inch (line 34) and 1/160 of a defect per square inch (line 36). Typically, color filters have dimensions of approximately 200 pixels per/inch (wherein 5 mils is equivalent to 127 microns). Panels having as few as three to four defects per ten square inch diagonal panel are considered unusable according to industry standards. Hence, it is not surprising that typical yields of current color filter manufacturing processes have heretofore been on the order of 50% and with repair approximately 70%.
Because of its high color quality, and despite attempts to improve its low yields, photolithography remains the process of choice in the production of color filters for visual displays. This being the case, there remains a need for a manufacturing process that produces color filters having good resolution and color quality in high yields.